Design, evaluation and verification of a small animal SPECT-CT workflow for biologic adaptive Radiotherapy

Introduction
In modern radiotherapy, tumour physiology plays an important role in treatment planning, verification and monitoring of treatment outcome. Molecular imaging, in combination with morphological imaging modalities and techniques such as CT and / or MRI, provides essential comprehensive information on tumour heterogeneity and risk profile.
In this context, preclinical imaging and research play a central role in the development of new imaging techniques for the planning of individualised radiotherapy.

The benefits of multimodal molecular imaging can only be assessed, tested and appropriately developed in a preclinical setting.

Material/Methods
The Molecubes modalities used in this master's thesis, micro CT, micro SPECT and micro PET, are operated and managed by the Competence Centre for Preclinical Imaging and Biomedical Engineering FH Wiener Neustadt.

Hypothesis: The acquisition time, the amount of radioactivity and the use of different reconstruction algorithms have a significant influence on the results of small animal SPECT-CT imaging.
The objectives of this work were to evaluate and determine a suitable acquisition and reconstruction protocol and to determine the dynamic range and system sensitivity
The following experimental set-up was set up to determine the dynamic range and, as a result, the sensitivity of the device:
A 20 ml syringe was used to mimic the volume of a mouse. 16 different concentrations of [99mTc] dissolved in sodium chloride were measured at least three times (n> 3) for acquisition times of 5, 10, 20, 30 and 40 minutes.
A 200 ml syringe was used to mimic the volume of a rat. Thirteen different concentrations of [99mTc] dissolved in sodium chloride were measured at least three times (n> 3). The measured data were corrected for radioactive decay.
The mean of the three measured concentrations and the standard deviation were calculated for both the mouse and the rat phantom.
Regression analyses were performed with the measurement results of the mouse phantom (5 different scan times) and the rat phantom (1 scan time). The values used for various regression analyses were selected on the basis of a best-fit scenario.
Results/Discussion
Looking at the results obtained from the 20-minute measurement with the µ-SPECT, the sensitivity of the device reaches its maximum at a concentration of 7.6167 MBq / ml (20 ml) and 0.9874 MBq / ml (200 ml). From this it can be deduced that the higher the sensitivity of the system, the fewer MBq/mL are required to obtain a valid signal.
Values outside the dynamic range do not improve the image quality. They therefore lead to a higher radiation exposure of the animal, a higher probability of incorrect measurements and, in the worst case, to a completely incorrect signal output. This evaluation is essential for future in-vivo measurements to ensure valid data is obtained.
Conclusion
During the evaluation of the set-up process, which serves as a tool to determine the sensitivity of the system, the influence of the acquisition time, the amount of activity and the use of different reconstruction algorithms on the SPECT-CT image quality was verified.
As the measured parameters have a major influence on the signal output, further measurements such as SNR, SBR, coefficient of variation etc. are planned.